A touch sensitive display typically includes a cathode ray tube monitor and a transparent, touch sensitive overlay which is attached to the face of the monitor. Such displays are usually part of a system including a small processor, such as a personal computer, and a source of external video, such as a video disk player or a video cassette recorder.
To use a touch sensitive display for a particular purpose, a program is written for execution in the processor. The program defines the response of the system whenever the screen is touched at a particular point. Depending on the program, a screen touch may cause video information to be retrieved from the external video source for display on the screen, either alone or in combination with graphics information generated by the processor. A screen touch may also result in the display of graphics information only.
Touch sensitive displays are not per se new. Such displays have been implemented using a number of different technologies. In one known technology, transparent conductors are formed in two slightly separated layers of transparent, flexible material. By "scanning" the conductors in one of the layers with sequenced voltage pulses and by noting at which conductor in the other layer on which the voltage appears, the horizontal (X) and vertical (Y) coordinates of the touched location can be determined. The X-Y coordinates represent a user response which is employed by the processor to determine the next system action.
Another technology used for touch input displays also has two slightly separated flexible layers but measures the capacitance at particular points on the touch sensitive surface. If one of the layers is touched, bringing two spaced conductors closer together, the capacitance will change at the touched location.
Another technology which has been suggested for touch input displays requires that a transparent, substantially rigid push plate be supported primarily by at least three force sensitive transducers, such as piezoelectric transducers. When the push plate is touched, the almost imperceptible movement of the push plate towards the CRT will exert pressure on the piezoelectric transducers, causing each to generate an electrical signal having a level proportional to the amount of force exerted on the transducer. By comparing the relative forces exerted on the various transducers, the location at which the screen was touched can be determined mathematically. Piezoelectric touch screens are considered to be less expensive to manufacture and yet more durable in normal use than other known types of touch screens.
The mechanical requirements for a piezoelectric touch screen display are not, however, simple. The piezoelectric transducers must be incorporated into an assembly which is substantial enough to maintain the alignment of the touch screen components relative to the face of the CRT. The assembly must also be substantial enough to withstand the repeated pushes which will occur as part of the normal system operation.
At the same time, the mounting assembly must not itself inhibit the movement of the push plate toward or away from the face of the CRT.